Flat magnetic core

ABSTRACT

A toroidal tape core is produced from magnetic sheets ( 1 ) which may have slits ( 4 ). In order to improve the behavior of the toroidal cores ( 3 ) at high frequencies, the magnetic sheets ( 1 ) have a high surface roughness. The surface roughness of each magnetic sheet ( 1 ) is at least equal to the skin penetration depth at the frequency being used.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention concerns a component of low overall height for circuitboards having a magnetic region formed by at least one layer made of asoft-magnetic material.

2. Description of the Related Art

A component of this type is known from U.S. Pat. No. 5,529,831. Theknown component is produced by applying insulator films, conductorfilms, and a magnetic film onto the substrate. A typical sputteringprocess is used to apply these films.

A disadvantage of this type of component is that it can only be producedwith the aid of a costly thin-film process. In addition, depending onthe process, only low film thicknesses in the range of a few μm can beproduced. The cross-sections of the magnetic regions produced with theaid of this process are correspondingly small. A further disadvantage isthat with this type of component, the windings must also be producedwith the aid of a costly thin-film process.

SUMMARY OF THE INVENTION

Proceeding from this prior art, the object of the invention is to createan easily producible component of high inductivity for use on circuitboards.

This subject is achieved according to the invention in that the magneticregion is formed by at least one soft-magnetic sheet. The surfaceroughness of each sheet is at least equal to the skin penetration depthat the usage frequency.

Magnetic sheets can typically be produced with thicknesses in the rangefrom 10 to 25 μm. If they are stacked on top of one another,significantly larger cross-sections of the magnetic region than those ofmagnetic regions produced in thin-film processes thus result. As aconsequence, the inductivity of a component equipped with this type ofmagnetic region is relatively high. Nonetheless, the component accordingto the invention has a low overall height and is therefore also suitablefor SMD technology in this regard. It is particularly favorable for highfrequency applications that the surface roughness of each sheet is atleast equal to the skin penetration depth at the usage frequency.

Further embodiments and developments are the object of the dependentclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, exemplary embodiments of the invention are describedwith reference to the attached drawing.

FIGS. 1A to 1C show various embodiments of magnetic sheets which couldbe considered for usage in a magnetic region of a component;

FIG. 2 shows a perspective view of a sequence of magnetic sheets stackedon top of one another;

FIG. 3 shows a sequence of magnetic sheets stacked on top of one anotherwhich are provided with a gap;

FIG. 4 shows an exploded view of a magnetic region formed from magneticsheets with an offset gap;

FIG. 5 shows a cross-sectional view of a stack of magnetic sheetsembedded in a plastic trough;

FIG. 6 shows a cross-sectional view through a stack of magnetic sheetsenclosed by a polymer film;

FIG. 7 shows an illustration which clarifies the definition of surfaceroughness;

FIG. 8 shows a schematic illustration of the course of the eddy currentsfor a smooth tape;

FIG. 9 shows a schematic illustration of the course of the eddy currentsfor a rough tape; and

FIG. 10 shows a diagram of the frequency response of components made ofsmooth and rough magnetic sheets.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS OF THE INVENTION

Various embodiments of a magnetic sheet 1 are illustrated in FIGS. 1A to1C. The magnetic sheet 1 illustrated in FIG. 1A has a circular ringshape. In contrast, the magnetic sheets 1 from FIGS. 1B and 1C have aring shape with rectangular contours. The magnetic sheets 1 are, forpractical purposes, produced from an amorphous or nanocrystalline alloy.Amorphous alloys based on iron are, for example, known from U.S. Pat.No. 4,144,058. Amorphous alloys based on cobalt are, for example, knownfrom EP-A-0 021 101. Finally, nanocrystalline alloys are described inEP-A-0 271 657. Thin sheets with a typical thickness of 10 to 25 μm, orsometimes, greater or lesser thicknesses, can be produced from thematerials mentioned. The ring-shaped magnetic sheets 1 can then bestamped out of the thin sheets.

The stacked magnetic sheets 1 result in a toroidal core 3, asillustrated in FIG. 2, with the thickness of the magnetic sheets 1 beingexaggerated in FIG. 2 in comparison to the diameter, as the diameter ofmagnetic sheets 1 is in the range of a few millimeters, while thethickness of, the magnetic sheets 1 is in the range of 10 μm.

The magnetic sheets 1 can be glued to one another to increase thestrength of the toroidal core 3. For high frequency applications, it isalso practical for damping of eddy currents to insulate the magneticsheets 1 from one another on one or both sides by the application of aninsulator film. The adhesive film can assume the task of an insulatorfilm at the same time.

In order to adjust the magnetic properties of the toroidal core 3, aslit 4 is produced in the toroidal core 3 illustrated in FIG. 3, whichshears the hysteresis loop. In the exemplary embodiment illustrated inFIG. 3, the slit 4 is produced after the stacking of the magnetic sheets1 and the gluing of the magnetic sheets 1.

In contrast, in the exemplary embodiment illustrated in FIG. 4, themagnetic sheets 1 are first individually provided with the slit 4 andthen stacked on one another and glued to one another. The production ofthe exemplary embodiment illustrated in FIG. 4 is more costly than thatof the exemplary embodiment from FIG. 3, but the toroidal core 3 fromFIG. 4 has a higher mechanical strength.

According to FIG. 5, it is provided that the toroidal core 3 be placedin a trough 5 manufactured from plastic to protect the toroidal core 3from mechanical damage. The trough 5 can then be wound with a windingthrough an inner hole 5′, without danger of the toroidal core 3 formedby the magnetic sheets 1 being damaged during winding.

In addition, there is the possibility of enclosing the toroidal core 3with a polymer film 6. This polymer film 6 is, for practical purposes, apolymer film precipitated from the gaseous phase, for example apolyparaxylene. This process has the advantage that the gaseous polymermaterial penetrates into even the smallest cracks and that in this waythe magnetic sheets 1 are also mechanically bonded to one another,without the magnetic sheets 1 being mechanically strained. A mechanicalstrain can, due to magnetostriction, disadvantageously change themagnetic properties of the magnetic sheet 1.

It is further advantageous for high frequency applications if thesurface roughness R_(A) of the magnetic sheets 1 is approximately equalto the skin penetration depth δ_(skin) at the usage frequencies.

The definition of the peak-to-valley depth is explained in the followingwith reference to FIG. 7. In this case, the x-axis is parallel to thesurface of the body whose surface roughness R_(A) is to be determined.The y-axis, in contrast, is parallel to the surface normal of thesurface to be measured. The surface roughness R_(A) then corresponds tothe height of a rectangle 7 whose length is equal to a total measurementpath I_(M) and which is equal in area to the sum of the surfaces 10enclosed between a roughness profile 8 and a center line 9. Thetwo-sided surface roughness R_(A rel) relative to the thickness of themagnetic sheet 1 then results according to the formula

R _(A rel)=(R _(A upper side) +R _(A lower side))/d,

with d being the thickness of the magnetic sheet 1.

The surface roughness R_(A) of the magnetic sheets 1 then affects thelength of the current paths, which determine the eddy currents. If theskin penetration depth δ_(skin) is less than half of the sheet thicknessat the usage frequencies, the currents flowing in the magnetic sheet 1are thus predominantly restricted to a boundary layer of the magneticsheet 1 with a thickness equal to the skin penetration depth δ_(skin).If the surface roughness R_(A) of the magnetic sheet 1 is then in therange of the skin penetration depth δ_(skin), the eddy currents mustfollow the surface modulated by the surface roughness R_(A), which leadsto lengthened current paths and therefore to a noticeably increasedspecific resistance. However, an increased eddy current limitingfrequency also results from this.

These relationships are illustrated in FIGS. 8 and 9. The windingcurrents 11 flowing in an outer winding produce eddy currents 12 in themagnetic sheet 1 in a surface region with a thickness equal to the skinpenetration depth δ_(skin). If the surface roughness of the magneticsheet 1 is then greater than the skin penetration depth δ_(skin),lengthened current paths result for the eddy currents 12, which leads toan increased eddy current limiting frequency.

The surface roughness selected can, however, not be arbitrarily large,because the magnetic sheets 1 can, in the extreme case, have holes,which strongly reduces the permeabilities achievable.

In FIG. 10, the influence of the surface roughness on the frequencydependency of the permeability μ described is illustrated with referenceto measurement results. The magnetic sheets 1 measured are magneticsheets 1 made of an alloy with the composition(CoFeNi)_(78,5)(MnSiB)_(21,5). A dashed curve 13 illustrates thedependence of the permeability μ on the frequency f at a total surfaceroughness of 2.1% relative to the thickness of the magnetic sheet 1. Asolid curve 14 further illustrates the dependence of the permeability μon the frequency f at a total surface roughness of 4.7% relative to thethickness of the magnetic sheet 1. It can be clearly seen that the eddycurrent limiting frequency is displaced toward higher values by thegreater surface roughness. It has been proven to be favorable if thetwo-sided surface roughness of the upper and lower sides is >3% relativeto the thickness of the magnetic sheets 1.

In the following, the advantages of the toroidal core 3 produced fromthe magnetic sheets 1 are described with reference to an example. Areactor used in telecommunications is to serve as the example. For thistype of reactor, an A_(L) value of 1 μH is required in the flattestpossible structural shape. The inductivity L is A_(L)×N² in this case,with N being the number of windings. The typical usage frequencies of areactor of this type are in the range of 20 kHz to 100 kHz, or higher insome cases. The smallest ferrite core commercially available at thistime is a MnZn-ferrite toroidal core from the firm Taiyo Yuden with anouter diameter of 2.54 mm, an inner diameter of 1.27 mm, and a height of0.8 mm. The material AH 91 used for production of the MnZn-ferritetoroidal core has an initial permeability of μ=10,000.

If an amorphous cobalt-based alloy with the compositionCo_(62,35)Fe_(3,92)Mn_(1,14)Si_(9,72)Mo_(0,40)B_(2,46), which has aninitial permeability μ=50,000, is used, an A_(L) value of 1 μH can beachieved with a significantly smaller toroidal core 3. For example, thetoroidal core 3 with an outer diameter of 2.54 mm, an inner diameter of1.8 mm, and a height of 0.4 mm could be considered. This toroidal core 3has an inner hole which is twice as large as that of the ferrite core,which allows either more turns or turns with an enlarged conductorcross-section.

The same A_(L) value can also be achieved with the toroidal core 3 withan outer diameter of 4.0 mm, an inner diameter of 2.85 mm, and anoverall height of 0.4 mm. This toroidal core 3 has an inner hole whichis larger than that of the ferrite core by a factor of 5.

Conversely, with the same outer and inner diameter, i.e. an outerdiameter of 2.54 mm and an inner diameter of 1.27 mm, an overall heightof 0.2 mm is sufficient to achieve an equal A_(L) value.

If material with even higher initial permeabilities is used, for examplean alloy with the compositionCo_(61,06)Fe_(4,21)Si_(9,43)Mo_(2,93)B_(2,35), which has an initialpermeability of μ=80,000, the overall height of the toroidal core can bereduced further. A toroidal core 3 made of the alloy with thecomposition Co_(61,06)Fe_(4,21)Si_(9,43)Mo_(2,93)B_(2,35), which has aninitial permeability μ=80,000, only requires an overall height of 0.125mm with an outer diameter of 2.54 mm and an inner diameter of 1.27 mm toachieve an A_(L) value of 1 μH. The toroidal core 3 manufactured fromthis alloy has an overall height which is smaller by a factor of 6.4than the ferrite core.

A further possible application is the use of the toroidal core 3 as theS_(o)transformer in PCMCIA cards. In card type I, S₀ transformers withan overall height of 2.2 mm are necessary so that the permissibleoverall height of 3.3 mm for a PCMCIA card is not exceeded. Taking intoaccount the winding and the housing walls, a maximum overall height of 1mm remains for the toroidal core 3. To achieve the required inductivityof approximately 5 mH at 20 kHz, for example, a toroidal core 3 with anouter diameter of 8.6 mm, an inner diameter of 3.1 mm, and an overallheight of 1 mm is necessary. The toroidal tape cores used for thispurpose until now are very mechanically sensitive and can therefore onlybe produced with a high rejection rate. For example, one problem is thehigh winding offset, due to which the core height is not met. Incontrast, the toroidal core 3 can easily be produced with highdimensional accuracy.

Linear hysteresis loops with low losses and high permeability can beachieved through suitable heat treatment in an external magnetic fieldby the use of the amorphous or nanocrystalline alloys. In addition, dueto the naturally insulating surface film of these alloys, it is notnecessary, in contrast to crystalline alloys, to insulate the magneticsheets 1 from one another by an additional insulating film. In addition,in comparison to crystalline alloys, the amorphous or nanocrystallinealloys have a higher specific resistance, which leads to higher eddycurrent limiting frequencies. Depending on the production, the amorphousand nanocrystalline alloys also have a natural surface roughness to agreater or lesser degree, which can, however, be increased further bygrinding or etching. The thickness of the magnetic sheets 1 is between 5and 40 μm. In the extreme case, the toroidal core 3 is formed by onesingle magnetic sheet 1. In this way, extremely low overall heights canbe achieved simultaneously with favorable high frequency behavior.

What is claimed is:
 1. A component of low overall height for circuitboards having a magnetic region formed by at least one layer made of asoft-magnetic material, the magnetic region comprising at least onesoft-magnetic magnetic sheet having a surface roughness at least equalto skin penetration depth at usage frequency.
 2. The component accordingto claim 1, wherein the at least one magnetic sheet is produced from ananocrystalline or amorphous alloy.
 3. The component according to claim2, wherein the surface roughness of the at least one magnetic sheetis >3% relative to its thickness.
 4. The component according to claim 1,wherein the magnetic region is formed by multiple magnetic sheets gluedto one another.
 5. The component according to claim 1, furthercomprising more than one magnetic sheet, wherein the magnetic sheets areinsulated from one another by insulating intermediate films.
 6. Thecomponent according to claim 1, wherein the at least one magnetic sheetis ring-shaped.
 7. The component according to claim 6, wherein the atleast one magnetic sheet is generally ring-shaped and has slits.
 8. Thecomponent according to claim 7, wherein the slits are positioned on topof one another.
 9. The component according to claim 7, wherein the slitsare positioned at offset angles.
 10. The component according to claim 1,wherein stacked magnetic sheets are embedded in a plastic trough. 11.The component according to claim 1, wherein magnetic sheets are stackedon one another and enclosed by a polymer film.